The broad objective of the project is to study growth characteristics of rickettsiae in cell cultures. In collaboration with other investigators, data obtained are applied to differentiation and classification of rickettsiae, their routine production for serologic, immunologic, and biochemical studies, and studies on their maintenance in nature. Current emphasis is on strains of the spotted fever group and certain unclassified strains, particularly those which grow poorly in embryonated eggs. Standard tissue culture procedures are used, including infectivity titrations by the plaque method. Inocula are frequently centrifuged onto cell monolayers when infection can be enhanced thereby. Different host cells and environmental conditions are tested to find combinations which provide optimum rickettsial growth. The project also includes preparation of purified and semi-purified concentrates of cell culture-grown rickettsiae for use as antigen and development of methods for such purification.